I am no expert by any means and am researching this myself. There seems to be more opinions than definitive answers (which never sets my mind at ease lol)
The common ground is that 24 hours is the standard. I am personally leaning towards believing that this timeframe exists not because the vaccine is less effective, but because that is the timeframe the chick leaves the incubator or begins drying off and taking steps away from mama hen. The vaccine is teaching the baby chick's body to defend itself, and I feel it could learn that on day 7 or 10 just as easily as on day 1.
So if the key is exposure, I suppose the answer depends on how old your chicks are and what their exposure level has been. Keep in mind I've read several things pointing to the fact that you can introduce your flock to MD simply by returning home from visiting a friend with chickens that are carriers. This is important because even if your brooder is inside, you could have exposed them to MD before they see sunlight.
That said, I have not read anything where the vaccine can be harmful to chicks older than 24 hours. So, for me, I've decided to spend the $30 and hope for the best on mine that hatched on Easter Sunday. They may be 2 weeks old when I give it, but worst case scenario is that it doesn't do any good.
If someone with more knowledge and experience disagrees, stop me now! LOL